The present invention relates to a large power semiconductor device having a self-protection feature. In particular, the invention is directed to a large power switching transistor device provided with a junction type field effect transistor (J-FET) acting as a protection circuit.
Recently, great progress has been achieved in the fields of variable motor control and automatic control of various machines. Also, the industry has witnessed the advent of power transistors with greater breakdown voltages. As a result, the demand for large power switching transistors such as a Darlington transistor is sharply increasing. Under the circumstances, it is a serious concern in this field to develop large power switching transistors having a high breakdown voltage while permitting the use of a relativley high voltage (e.g., 480 volts AC), and operating over a wide area of safety operation (ASO). ASO is the range of Vce voltages which can be applied to the transistor without a breakdown occurring.
If an accident occurs during the operation of a large power switching transistor (e.g., load short-circuiting, power surges, power source fluctuation etc.), a high voltage is applied between the collector and the emitter of the transistor. A large current will suddenly flow through the transistor and the transistor will be instantly destroyed. As a result, it is customary in the prior art to provide large power switching transistors with a protection circuit for protecting the transistor from destruction in the event of an accident. Such prior art circuits substantially increase the ASO during that time. It should be noted that the ASO will decrease in proportion to the continuous time the transistor is operating. Thus, it is necessary to provide some method in the prior art to prevent or limit breakdown as the transistor continues to operate.
FIG. 1 shows a conventional protection system for a power transistor Q1. A protective switch Q2, operable by the collector voltage of Q1, is connected between the base and emitter of Q1. When an excess voltage is applied between the collector and emitter of Q1, protective switch Q2 is turned on to short-circuit the base and emitter of transistor Q1. As a result, an excess collector current is prevented from flowing, thereby preventing the destruction of transistor Q1.
FIG. 2 is a graph showing the static characteristics of transistor Q1 and its protective switch Q2. The collector-emitter voltage V.sub.CE of Q1 is plotted on the abscissa, while the collector current I.sub.C is plotted in the ordinate. A.sub.1 to A.sub.3 represent the change of I.sub.C relative to V.sub.CE for each base current I.sub.B. Straight line b-b' represents the load characteristics; point b denotes the conductive state of the transistor and the point b' denotes its nonconductive state. The curves represented by the dashed lines show the characteristics of a transistor Q1 which has not been provided with a protective switch such as switch Q2.
The conventional protection system described above utilizes protective switch Q2 to force transistor Q1 into the cut off region (i.e., values of V.sub.CE greater than or equal to V.sub.CO -FIG. 2). This system, however, is defective since at V.sub.CE voltage values above V.sub.CO, transistor Q1 does not operate. As seen from FIG. 2, curves A.sub.1 -A.sub.3 have collector current (I.sub.C) values equal to zero above V.sub.CE voltage values greater than V.sub.CO. Therefore, it is difficult, if not impossible, to operate transistor Q1 at point b or other non-zero collector current values because switch Q2 is constantly on and collector current I.sub.C is thereby prevented from flowing.
In order to permit Q1 to pass some collector current while switch Q2 is operating, it is necessary to set the bias point b" within the narrow range R. In particular, b" must be set between the operating points R.sub.1 and R.sub.2. R.sub.1 represents the value of V.sub.CE which provides some collector current for all curves A.sub.1 -A.sub.3. R.sub.2 represents the lowest value of V.sub.CE where breakdown occurs.
In order to set the bias point b" in this fashion, the power source voltage for driving the transistor must be determined by the static characteristics of the transistor device including the protective switch Q2. As a result, a substantial and practical restriction is imposed on the setting of the power source voltage. In addition, since the operating characteristics vary among the individual protective circuits, the setting for the source voltage must be changed for each transistor. Thus, it is very difficult to design the power source circuit. Additional difficulty is presented since the bias voltage must be sufficiently stabilized in order to enable the protective circuit to operate normally despite the existence of source voltage fluctuations. It is extremely difficult, however, to stabilize the bias voltage due to the fluctuations.